Increased environmental awareness in recent years has led to a demand for increased tire fuel economy. There is also a strong demand in the industry for rubber composition for use in tire side walls that yield not only improved breakdown resistance but also improved fuel economy.
An example of a method of reducing the amount of heat generated by a rubber composition for use in tire side walls is to reduce the amount of filler included therein. However, such means leads to reductions in hardness and tire strength. Another method is to use a molecular weight-reduced terminal-modified diene rubber and large amounts of modifiers in order to suppress heat generation, but this method also presents the problem of filler over-dispersion leading to reductions in hardness. Meanwhile, increasing the compounded amount of vulcanizing system and increasing crosslinking density in order to suppress reductions in hardness allows for a balance of hardness and reduced heat generation, but degrades flexural fatigue resistance.
Japanese Unexamined Patent Application Publication No. 2006-063287 discloses a technique of adding carbon black having a specified specific surface area to a diene rubber component containing a specified butadiene rubber. However, the technique disclosed in Japanese Unexamined Patent Application Publication No. 2006-063287 does not satisfactorily demonstrate high levels of hardness reduction suppression, fuel economy, cut resistance, and flexural fatigue resistance.